How to add new brush set for sculpting?
This is Happening Because this Add-on is Not Updated to 2.8 and blender 2.8's Python API is Changed and this is Compatible for its Older Versions It May Be Compatible with blender 2.79 or older
In simpler words this Add-on is Older This Should Be Updated By its Developers for blender 2.8 
But If you Still wan't to use those Brushes I Have an Idea It May work or May Not
But still Idea is -
First of All Unzip the Add-on File and 
It May Contain a Blend File in It or Some Images


If it is a Blend File then go to blender and File> Append > Click on That Blend File > Brushes > select all > Hit Append.
If there are Images in It in format of tiff, targa or any format then Just Go to Blender and Then Textures Tab and Click New > Set The Type To Image > and then select that Brush Image File and Just Switch To Sculpting and Slect That Image Texture.
I hope You UnderStood